This application claims the benefit of EP Patent Application 08290728.8 filed Jul. 24, 2008, entitled, “Control of the Properties of Cement Slurries of Normal Densities With Optimized Polymers Combination.”
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present invention broadly relates to well cementing. More particularly the invention relates to cementing compositions comprising a mixture of cellulosic polymer and AMPS monomer based polymer as well as methods for using such compositions to service a wellbore.
When a well such as an oil or gas well has been drilled, it is often desired to isolate the various producing zones from each other or from the well itself in order to stabilize the well or prevent fluid communication between the zones or shut off unwanted fluid production such as water. This isolation is typically achieved by installing a tubular casing in the well and filling the annulus between the outside of the casing and the wall of the well (the formation) with cement. The cement is usually placed in the annulus by pumping slurry of the cement down the casing such that it exits at the bottom of the well and passes back up the outside of the casing to fill the annulus. Various additives can be added to the cement to enhance properties of the slurry or of the set cement, for example fluid-loss control agent, foaming agent, dispersant, retarder, and/or accelerator.
Hydroxyethyl celluloses (HEC) have been used for many years as fluid-loss control agents in oilwell cement slurries. As such, various grades having different molecular weight and ethoxylation (degree of substitution and molar substitution) cover slightly different ranges of application in terms of temperature and slurry density. They all suffer from viscosifying and retarding too much the cement slurry, thus their use is limited to temperatures typically lower than 85° C. and to lighter weight slurries. By way of examples, a low molecular-weight HEC, is used as a fluid-loss control agent in cement slurries having a Solid Volume Fraction (SVF) of about 35% to about 60%. A low/medium molecular-weight HEC is used as a fluid-loss control agent in cement slurries having a SVF of about 20% to about 30% (i.e., extended slurries). And a high molecular-weight HEC, is primarily used as an extender in extended cement slurries. Last one also provides some fluid-loss control.
Also, co- or ter-polymer containing AMPS and one or several vinyl or acrylic co-monomers (like Acrylic acid, Methacrylic acid, Acrylamide, NN dimethylacrylamide, N vinylpyrrolidone, NN dimethylformamide, N-vinyl-N-methylacetamide, ACMO . . . ) have been initially developed in the early 1980's and improved in the late 1990's/early 2000's. These synthetic polymers can be either long chain polymers or short-chain elements grafted onto a natural polymeric backbone (tannin, humic acid, causticized lignite . . . ) for improving the ecotox properties. These synthetic polymers are much more expensive than the HEC derivatives and are supposed to provide fluid loss control at much higher temperatures. However some of these synthetic polymers also viscosify significantly the cement slurries, which limit seriously their temperature range. Other synthetic polymers on the other hand does induce severe slurry instability and particle settling.
US2004/262000 discloses a method for cementing in a subterranean formation providing a inter alia an acrylamide copolymer and a hydratable polymer. The ratio copolymer to hydratable polymer varies between 10:1 to 22:1. Then, US2003/008779 relates to a well treating fluid comprising an interjacent polymer complex which is made of HEC and a water soluble polymer at least partially derived from 2-acrylamido-2-methyl propane sulfonic acid. Another patent application, US2004/226483, aims at a cement composition containing the interjacent polymer of US2003/008779.
Therefore, a solution to decouple the fluid-loss control and the slurry viscosity is needed for cement slurry and particularly one, which would be effective over a large range of temperatures, which would allow a reduced concentration in AMPS and which would not imply a lengthy process such as forming an interjacent complex polymer.